1. Field of Invention
The present invention relates to a differential amplifier circuit.
2. Description of the Prior Art
A differential amplifier is suitable for amplifying a faint signal such as the output of a sensor. It is able to effectively reduce the adverse effects of noise.
The forward threshold voltage of an Si bipolar junction transistor is determined by a band gap which is a physical constant of Si, therefore an Si bipolar junction transistor is not significantly affected by fluctuations in manufacturing process. With Si bipolar junction transistors being used as active input elements for differential amplifiers, it is possible to achieve a small offset voltage (several tens .mu.V). Accordingly, silicon (Si) bipolar junction transistors have been generally used as active input elements for differential amplifiers. However, if ambient temperature becomes considerably higher than room temperature, excited electrons jump from a valence band into a conduction band in such an Si bipolar junction transistor, thus preventing normal operation.
There have been various attempts to construct a differential amplifier from a transistor of a material other than Si, such as GaAs or the like, where the material has a larger band gap than Si so that the transistor can withstand a high temperature environment. At present, field effect transistors (FETs) using new materials, such as GaAs or the like, are widely used.
FIG. 4 (Prior Art) is a circuit diagram showing a differential amplifier using GaAs MESFETs. In FIG. 4, reference numerals 1 and 2 designate input terminals respectively; 3 and 4, output terminals; 5 and 6, GaAs MESFETs; 7 and 8, load elements; 9, a constant-current regulated power source; 10, a high potential side power supply terminal; and 11, a low potential side power supply terminal.
In a differential amplifier as shown in FIG. 4, the difference in threshold voltages between FETs 5 and 6 causes an equivalent input offset voltage. The threshold voltage of an FET depends on the concentration of impurities doped below a gate electrode. Therefore, the threshold voltage varies extremely depending on the manufacturing process. Accordingly, in the case of a differential amplifier as shown in FIG. 4, the equivalent input offset voltage of several tens mV is often present. Then, the output voltage becomes a value of the equivalent input offset voltage multiplied by a gain of the differential amplifier thereby resulting in an occurrence of an extremely large error.